1. Field of the Invention
The invention relates generally to nondestructive testing and in particular to the use of a rotating eddy current probe to inspect steam generator tubes.
2. General Background
Steam generators used in energy production are generally of the straight tube or U-tube design. Coolant from the nuclear reactor travels through the tubes and transfers heat to the secondary coolant in the steam generator. Due to the serious consequences which would result from a tube leak, the tubes are inspected for defects on a routine basis. These routine inspections typically use a bobbin eddy current coil.
Based on the routine exam, an advanced inspection technique carried out by the use of a coil in a rotating eddy current probe inside the tubes may be performed. The probe may be in contact with or near the surface of the tube and is rotated as it moves through the tube. This results in a helical inspection path for the coil. In the preferred method of inspection, the coil must be maintained in continuous close proximity with the tube's inner surface to obtain inspection results which accurately indicate the presence or absence of flaws in the tubing. In addition to problems encountered with surface irregularity, U-tube designs present higher frictional drag in the bent portion of the tube and a cross section which is more oval in shape than round. Devices for detecting defects in tubular members which are known to applicants include the following.
U.S. Pat. No. 4,675,604 discloses a device for detecting defects which includes an electromagnet for magnetizing the internal surface of a tubular member. The electromagnet is radially supported on rollers.
U.S. Pat. No. 2,684,464 discloses a hollow shaft which is mounted on bearing members so as to be rotatable independently of the bearing members. A detector head is mounted to a support member attached to the shaft so as to be in close proximity to the inner bore surface of the tubular member to be inspected.
U.S. Pat. No. 3,443,211 discloses the use of a magnetic inspection device with a non-contacting caliper having no movable parts that is pulled along the surface of the inspected member.
U.S. Pat. No. 3,472,319 discloses an electromagnet carried by a resilient distortable carriage constructed to urge the electromagnet toward the inner wall of the tube.
U.S. Pat. No. 3,091,733 discloses the use of a standardizing magnet and a stationary detector that detects flaws in the structure being examined by magnetic force changes.
U.S. Pat. No. 4,625,165 discloses a rotating head which carries a transversely slidable sensing member. Readings are taken only as the probe is pulled back towards the open end of the tube through which it was inserted.
An inspection device which overcomes the shortcomings in the present art of difficult rotation, difficulty in tracking the curvature of the tube inner surface, and tendency of the inspection coil to rock away from the inner surface of the tube in varying geometry and frictional forces is needed.